Electrochemical capacitors are a class of high rate energy storage/discharge devices, which use electrolytes and electrodes of various kinds, in a system similar to that of conventional batteries. Electrochemical capacitors, like batteries, are essentially energy storage devices. However, unlike batteries, they rely on charge accumulation at the electrode/electrolyte interface to store energy. Charge storage in capacitors therefore, is a surface phenomena. Conversely, charge storage in batteries is a bulk phenomena, occurring within the bulk of the electrode material.
The energy (E.sub.s) stored by electrochemical capacitors can be described by the following formula: EQU E.sub.s =1/2CV.sup.2
Where C is storage capacitance, and V is cell voltage. It is therefore possible to increase the energy stored in the electrochemical capacitor by increasing the cell voltage (V), the storage capacitance (C), or both.
Several methods are currently known in the art for increasing the amount of energy stored in an electrochemical capacitor. One such method is to increase the surface area of the active electrode. To this end, porous, spongy, and foam substrates have been proposed and demonstrated to increase the storage capacity. High surface area electrodes result in an increase in storage capacitance, and thus increased stored energy. However, the energy density, i.e., energy per unit volume, is generally lower than for non-porous electrodes since the volume-to-area ratio for porous structures is generally high.
Another known approach for increasing stored energy involves using different types of material for fabricating the capacitor's electrodes. Carbon electrodes are used in the most popular commercial capacitors, while precious metal oxide electrodes are used in a relatively new class of capacitors known as pseudocapacitors. The phenomena known as pseudocapacitance was first identified by Professor Brian Conway. Pseudocapacitor devices are disclosed in, for example, Canadian Patent No. 1,270,296 issued to Dwight Craig.
Pseudocapacitors are surface energy storage devices. Under this general category, reside two subcategories of capacitors: double layer capacitors in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge; and true pseudocapacitors in which charge transfer between the electrolyte and the electrode occurs over a wide potential range, and is the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and electrolyte.
Double layer electrochemical capacitors are typically fabricated from relatively inexpensive materials, such as carbon. Capacitance of such electrodes is typically on the order of approximately 3-30 .mu.F/cm.sup.2. Conversely, pseudocapacitive devices demonstrating the redox mechanism are typically fabricated from precious metal oxide electrodes. These electrodes, while having higher capacitance (i.e. on the order of approximately 150-300 .mu.F/cm.sup.2) are prohibitively expensive.
Moreover, using similar materials for both electrodes of a capacitive device results in incomplete use of the electrodes. This is because, during the charge/discharge process, material on one of the electrodes will be reduced while the material of the other electrode is being oxidized. Thus, since both electrodes are fabricated of the same material, during cell discharge the cell voltage reaches zero volts when half of the material in each electrode is utilized. Hence only half the energy stored in the electrode is delivered.
Accordingly, there exists a need to develop an electrochemical capacitor which provides a high capacity per unit weight for energy storage without compromising the volume-to-area ratio of the cell. Moreover, in order to provide for reasonably priced commercial applications, the device should be fabricated of readily available, commercial materials, or at least minimize the use of exotic, high priced materials.